Polyoxyalkylene glycols are important starting materials for the production of elastic fibers, elastic construction materials and coatings. They are prepared by polymerizing tetrahydrofuran (referred to hereinbelow as “THF”) or by copolymerization of THF with oxiranes such as ethylene oxide or propylene oxide or with alpha,omega-diols in the presence of cationic catalysts. EP-A 126 471, for example, discloses the use of heteropolyacids as catalysts. This process makes polyalkylene ether glycols accessible in one stage, whereas other processes initially provide the esters of the polyoxyalkylene glycols which still have to be hydrolyzed to the polyoxyalkylene glycols before their use in the field of polymers.
The determination of a certain target molecular weight in the heteropolyacid-catalyzed THF polymerization by controlling the amount of, and appropriately metering in the amount present in the polymerization system of, proton-donating compounds such as water in the course of the polymerization via measurement of the electrical conductivity is disclosed by DE-A 41 08 047.
A batchwise process for preparing THF copolymers using alpha,omega-diols in the presence of a heteropolyacid is disclosed by JP-A 10-87811. In this process, a portion of the copolymerizetion solution is continuously withdrawn from the polymerization reactor and subjected to a process for water removal. After 12 h, the polymerization is stopped. For the repetition of the experiment four times as described, fresh catalyst is used each time. This is uneconomic, since fresh catalyst has to be dried for each new polymerization.
It has been found that the heteropolyacid phase of the copolymerization mixture changes depending on the phase separation time and duration for the recycling. Viscosity and color increase, which affects firstly the quality of the polymerization product, for example the color of the end product, and secondly the properties of the heteropolyacid, for example pumpability, emulsifiability, on-stream time, but also the catalytic properties.
It is an object of the present invention to make the copolymerization of THF with alpha,omega-diols in the presence of heteropolyacids more simple and economic by providing copolymers of a certain molecular weight and finding a means of repeatedly using and recycling the catalyst.
The novel process should additionally provide polyoxyalkylene glycols with incorporation rates of the diol comonomer of from 5 to 40% by weight, based on the copolymer.